SKU numbers explained: how to create, organize, and manage SKUs for ecommerce inventory

Every growing business starts with the best intentions for its inventory system. Your SKUs were once clear, following a logic everyone understood. 

But as your business expanded—new products, new variants, new team members—that clarity… faded. 

Take a look at your current inventory list. Can you easily make sense of every SKU? Are you finding old codes from people who are no longer with the business, or identifiers for products that have been retired or combined? 

Your SKU environment getting messier over time is a concern for growing ecommerce companies. The once-organized system feels less like a clean database and more like a patchwork and is no longer fit for your current size and scale.

This guide is not about making your first SKU (we already have a post on the basics). Instead, this is for people who already use SKUs and are running into problems—like too many variants, mismatches between channels, bundles that make stock counts confusing, and the growing sense that no one on the team really understands how the system works anymore.

Where SKU systems break

SKU problems don’t always reveal themselves as serious problems. They might show up as small issues, like a stuck order on Amazon or a mismatched count on Shopify.

But those small issues signal that your SKU system is no longer working as it should.

SKU systems often fail when variant names aren’t consistent and when marketplace-specific IDs are used instead of your main SKU.

This leads to outdated or made-up SKUs and bundle setups that often cause errors in part stock counts.

The 2026 Linnworks State of Commerce Operations report found that only 33.2% of UK retailers and 37.2% of US retailers rate their inventory visibility across channels as “excellent.” Most businesses know they have gaps, and as your business grows, these gaps can turn into bigger problems that make it hard to reconcile your inventory fully.

A format that survives growth

When people try to fix a SKU system, they often want to change the format. Sometimes that helps, but usually the real issue is the lack of clear rules for adding new SKUs or versions.

A good SKU format does three things: it provides enough information for people to understand it easily, it allows you to add new versions and product types without starting over, and it remains consistent so that different people create similar SKUs for similar products.

Most strong formats look something like:

[brand]-[category]-[product-id]-[variant]

For example, a Trail Co. men’s hiking shoe in olive, size 10, could be: TRL-HSH-201-OLV10. The brand uses three letters; the category (hiking shoes) has its own three-letter code; the product has a numeric ID; and the variant shows color and size.

The most important thing to remember is that you should stick to your own SKU naming convention. How you build it is less important than building it the same way every time. 

You run into problems when people break from the rules. For example, if color comes before size in some SKUs and after in others, sorting and reporting get harder. If size is sometimes a number, sometimes a letter, or sometimes written as “small,” the system can’t organize them properly.

Here are a few rules to employ:

• Parent-child structure: Use a main SKU for the product and separate SKUs for each version. The main SKU shows the base product, and the others show the specific versions. This is how Amazon, Shopify, and most marketplaces organize their data, and matching your catalog to this saves a lot of work later.
• SKU vs. attribute: Decide what counts as a SKU and what is just an attribute. Color and size usually count as SKUs because they affect stock. Marketing text and category labels usually do not. This choice affects how useful your reports will be.
• No changing data: Avoid putting information that changes inside a SKU. Pricing, supplier name, and promotions should not be part of a SKU. They change, but the SKU should stay the same.

SKUs that people can read easily help warehouse teams pick items, but they limit how many products and versions you can have. SKUs made only of numbers, like Amazon’s ASINs, can handle very large catalogs but are hard to understand without checking. Oftentimes, businesses end up with a mix–they begin with easier to read SKUs, but as they scale they are forced to switch to a number-based naming convention to handle the volume of products. 

Mapping SKUs across sales channels

As soon as you start selling on more than one channel, your SKU system has another job. It needs to match your internal catalog with the product identifiers used by each marketplace.

Amazon uses ASINs, FNSKUs, and Seller SKUs. eBay calls its identifier a Custom Label. Shopify maintains variant SKUs at the product level. Walmart relies on item IDs and GTINs, and TikTok Shop runs its own ID structure underneath. Each channel maintains its own product taxonomy, and each assumes you’ll map your SKUs cleanly into it.

In practice, sellers handle this in one of three ways.

The first way is to let each channel have its own SKU and match them up by hand. This can work if you only have one or two channels and a few hundred SKUs. Beyond that, it becomes too hard to manage mismatches.

The second is to use your master SKU as the Seller SKU on every channel and let the marketplace generate its own identifiers underneath. This is the cleanest setup, but it requires that your master SKU format is compatible with every channel’s requirements. Some marketplaces have character restrictions or length caps that constrain what you can use.

The third is to use an inventory management platform as the single source of truth, with channel-specific SKU mappings handled in the background. The platform maintains your master SKU internally and pushes the right identifier to each channel automatically. This is how most multichannel retailers past a certain size eventually operate.

The 2026 State of Commerce Operations report says 29% of UK and US retailers find syncing inventory across channels to be a big challenge. For UK retailers with four or more channels, this jumps to 58%. The main issue is usually matching SKUs. When channels don’t agree on stock, you might oversell, underprice, or get orders for items you don’t have.

Source BMX, a global seller across multiple marketplaces and direct-to-consumer channels, faced this problem before switching to Linnworks. Their co-founder Rich said they reached multimillion-dollar revenue targets in their first year after migrating, and they couldn’t have done it with their old system. The key change was moving away from separate inventory pools for each channel, manual reconciliation, and the constant risk of overselling during busy times.

Bundles and breakdowns

Customers see a bundle as one product, but your warehouse sees it as several. If your SKU system can’t handle both views, your stock counts will be off.

There are a few ways to manage bundles, and no single method works for everyone.

Option one is to assemble physical bundles ahead of time and treat them as separate SKUs with their own stock counts. This works for bundles that sell regularly and have fixed parts. The downside is you might hold extra stock, since parts are tied up in bundles that may not sell.

Option two is to use a virtual or “phantom” bundle SKU that reduces component stock when sold. Customers see one SKU, but behind the scenes, the system takes one of each part from stock and updates the counts. Most modern inventory systems support this, and it’s generally the better choice for bundles that change or kits made by sellers.

The third option, which causes the most problems, is not having a formal way to handle bundles. The team updates stock counts manually when bundles sell, either by hand or in batches. This can work with low sales, but as you grow, it often leads to selling more parts than you have on other channels while bundle orders stay unfilled.

The Linnworks platform handles bundles as a virtual setup, where one main SKU links automatically to the parts underneath and stock counts stay correct across channels even as bundles and parts sell. The important detail: when a part runs out, the system can replace it with an approved alternative instead of removing the bundle from listings. 

Who owns the SKU system

Most SKU systems do not have a clear owner. One person may have created the system, another keeps the spreadsheet, someone else adds new SKUs for new products, and a different person manages listings for each channel. No one is responsible for keeping the whole system consistent.

This causes common problems. New product lines stretch the SKU format beyond what the original creator planned. Old SKUs are not updated when categories change. When a product is discontinued, the SKU often stays active because people worry removing it might cause other problems.

The fix isn’t complicated. Ready? 

Pick one person who owns the SKU system end-to-end. 

Revolutionary, right?

That person:

• Approves new SKU creation against the documented format
• Audits the master catalog on a defined schedule (quarterly is enough for most operations)
• Decides when SKUs get retired and what “retired” really means in your system
• Owns the format documentation and updates it when the rules change

Most businesses skip the audit step. A SKU system is always changing—new products are added, variants grow, and suppliers change. If you don’t review the system regularly, it will slowly become less effective. Regular audits help you catch problems early.

On retirement: discontinuing a product is not the same as deleting its SKU. Most systems should keep historical SKUs intact for reporting, returns, and reorder analysis, but mark them clearly as inactive so they don’t pollute current operations. A SKU that’s been discontinued but still receives returns is alive in a way the current system needs to understand.

Signs your SKU system has stopped working

A few diagnostics. If three or more of these are true for your operation, the system needs structural work:

• Two team members create SKUs for similar products and produce noticeably different results
• A pull of all active SKUs shows more than 10% with zero sales velocity in the past 12 months
• You can’t reliably answer “how many SKUs do we sell on each channel” without exporting and de-duplicating
• Bundles or kits cause regular stock discrepancies during reconciliation
• New channel launches require manual mapping for the majority of SKUs
• Returns come in for products that don’t match any current active SKU

None of these problems are terrible on their own. Most businesses handle one or two for a long time. But when several problems add up, they create hidden costs in every part of your operation—like forecasting, reordering, adding new channels, and fulfilling orders. These costs grow over time.

A 60-minute SKU audit

If your system feels unreliable, here is a practical audit you can do this week. It does not need new software. It gives useful answers.

1. Export your full list of active SKUs. Sort by sales speed over the past 12 months. Find the bottom 10% by units sold. These are candidates to retire, combine, or check.
2. Use the same list and group by category code or prefix. If any category has fewer than three SKUs, ask if the code is useful or if those products should be grouped differently.
3. Check how variants are formatted. Pick five product groups and look at how their versions are labeled. If the format changes within a group, that is the first place to make consistent.
4. Match channel SKUs to your main list. For each channel, make sure every active listing has a matching active main SKU. Unmatched SKUs on either side are problems waiting to happen.
5. Check bundles. List every bundle SKU. Confirm the parts, the stock type (physical or virtual), and that the bundle is listed somewhere it can be sold.
6. Write down the format. Put in one document the rule for how new SKUs are made. Share it with everyone who creates SKUs.

The first time you do this audit, you will find more problems than you can fix immediately. That is okay. The goal is to make your SKU system clear enough so you can keep it organized going forward.

The next step

If you only do one part of this audit this week, export your bottom 10% of SKUs by sales speed. The list will probably be smaller than you expect, and deciding whether to keep, combine, or retire these SKUs will have the biggest effect on cleaning up your system. Start with this step. You can fix the format later.

FAQs

What’s the difference between a SKU and an ASIN?

A SKU is yours; an ASIN is Amazon’s. The SKU is the internal identifier you assign for inventory and operational tracking. The ASIN (Amazon Standard Identification Number) is the marketplace-level identifier Amazon assigns to every product in its catalog. Multiple sellers can sell against the same ASIN, but each seller has their own SKU.

How many SKUs is too many?

There is no fixed limit. What matters is how many active SKUs actually sell. If 30% of your catalog hasn’t sold in 12 months, you have extra complexity that does nothing. Cleaning up unused SKUs usually improves forecasting and report clarity more than adding new products.

Should I use the same SKU across all sales channels?

Generally, yes. Using your master SKU as the Seller SKU on every channel reduces reconciliation friction and makes inventory sync reliable. The exception is when a channel imposes character or length restrictions that your master format can’t meet. In that case, apply a uniform transformation rule rather than ad-hoc rewrites.

How do I handle SKUs for product variants?

Use a parent-child structure. The parent represents the base product, and children carry variant-specific identifiers (color, size, configuration). This aligns with how Amazon, Shopify, and most modern marketplaces structure their catalogs, and it makes bulk operations and reporting significantly cleaner.

What happens to a SKU when I discontinue a product?

Mark it inactive in your inventory system, but don’t delete it. Historical SKUs are useful for returns, reorder analysis, and historical reporting. The key is making sure inactive SKUs don’t pollute active operations: they shouldn’t appear in current stock counts, listings, or replenishment recommendations.

When should I move from spreadsheets to inventory software for SKU management?

The usual breakpoints are catalog size and channel count. Most operations can manage a few hundred SKUs on a single channel with spreadsheets. The signal it’s time to move is when reconciliation between channels starts taking real hours every week, when variants are proliferating faster than the format can absorb, or when bundle stock counts stop being trustworthy. For most multichannel sellers, that point arrives somewhere between 500 and 1,500 active SKUs.